When making electric lamps with a single pinch seal, and particularly electric lamp bulbs utilizing quartz glass or hard glass in which an electrode system is retained, it has been customary to reduce one end of an open glass tube in diameter, and seal a pump tube or exhaust tube thereto. The electrode system is introduced from the other end. The bulb is flushed with a flushing gas, such as argon while a pinch or press machine seals the electrode system at the end remote from the exhaust tube. The raw lamp bulb, including the exhaust tube, is then placed on a special machine stand. Fill substances, for example a halogen pellet such as an iodide pellet, mercury and the like are introduced through the pump tube. The fill gas may already be retained within the lamp bulb, for example if argon has been used as a flushing gas it may, at the same time, form the fill gas. The small tube at the end remote from the pinch or press seal, where the base will also be formed, is then tipped off, to melt the bulb shut.
The finished bulb will leave a small melt tip, which formed the pumping tube before it was tipped off. This pumping tube tip at the lamp vessel is undesirable; it has several serious disadvantages.
If the lamp is operated in a base-down position, and the lamp bulb is used with an electrode system which forms a discharge arc, the tip point is highly heated by the discharge between the electrodes. Any non-uniformities in the wall thickness may lead to deformation of the lamp vessel due to the high temperature and the high operating pressure, which may rise to about 50 bar. In extreme situations, the melting point or junction of the prior tip tube to the lamp vessel may become leaky or, if the lamp wall is thin at one place, the lamp vessel may burst. If the lamp is operated base-up, the cold spot temperature of a discharge lamp, and thus the color index of the light emitted by the lamp, will be determined, at least in part, by the distribution of the material at the region of the pumping tube tip, and the geometry thereof. Thus, the lamp may not meet specifications. Differential distribution of lamp bulb material in the vicinity of the exhaust tip also leads to optical distortion of the light. This is particularly undesirable if the lamp is to cooperate with reflectors requiring a predetermined light distribution therefrom.